In my opinion these stamps have much more historical value that the monetary. I'm not sure how rare they are, although found some info about similar stamps on the Net.
I saw one exactly the same stamp as yours (the first one from upper left, first link) and someone gave the price for it 40PLN in an auction (not known if someone bought it).
On the other site I found this:
One of your stamps, however a set with stamped postcard costs here even 800PLN (no bids though)
Maybe try to put something on this site.